Install and preserve /home partition

I want to install ubuntu 18.04 in my previous home partition with ubuntu 16.04.

I know that i need to format /root and preserve /home but i only want my documents(music, videos etc) not the hidden folders and settings.

just want a 'fresh install' and preserve my documents.

So i need to delete my old config files?
or are reset when i use the same username and password?
are there issues with this?

Why not save all your desired data to a portable drive, do a fresh install of 18.04, then copy your data from the portable drive to your new 18.04 folders?

because i do not have an external drive.

What I usually do is keep the previous OS in another partititon and copy only the files I need from one partition to the other.
I also left the old operating system as backup in case something goes wrong.

Since you already have a separate / and /home partition, you can proceed to use the installer and select Something else... to point to the existing partitions.

If you use the same username - use the live session to delete the dot files from the home partition before you reboot to your new install. CTRL+H will reveal them. They'll be re-created when you next login.

If you use a different username - the old home folder remains intact, in which you can move the folders to the new user after the new installation is finished.

:warning: This is assuming, of course, this isn't an encrypted home folder.

For guidance, @wolfman has written a guide on this type of installation: